Laser marking is a well known and important means for quickly and cleanly inscribing plastic surfaces with permanent informational indicia marks, such as date codes, batch codes, bar codes or part numbers, functional marks, such as computer keyboard and electronic keypad characters, and decorative marks, such as company logos. However, such laser marks are visible and readable only in lighted environments. The inability to read informational indicia on these and other items, such as clocks, emergency exit signs, safety information signboards, interior automobile control buttons, and the like, in a dark or dimly lit environment is a common problem in modern society. As used in the context of the invention, the term "indicia" refers to any laser mark whatsoever including, but not limited to, alphabetical characters, numbers, drawings, patterns, and the like.
It has been the practice to enable visibility of indicia in the dark by providing electrically powered illumination systems. Some illumination devices are self-contained, having a battery and light which clips onto the article to provide external illumination. Other systems rely on a light source within the article, the light being projected through the wall of the article to provide "back light" illumination of the indicia thereon or of the area immediately surrounding the indicia to provide contrast. Other devices utilize an optical conductor to transmit illumination from a light source to, for example, the keys of a key pad. Such illumination systems, however, whether battery powered or electrically wired, add bulkiness, weight, expense and maintenance costs to the item. Moreover, in cases of power outages, electrically wired systems are useless and many safety items, such as emergency exit signs, require back-up battery systems.
The use of phosphorescent materials in the production of luminescent articles is very well known, having been used for such diverse "glow-in-the-dark" items as golf balls, rubber shoe soles, many varieties of toys, safety helmets, safety tape for bicycle visibility, and the like. Phosphorescent phosphors are substances that emit light after having absorbed ultraviolet radiation or the like, and the afterglow of the light that can be visually observed continues for a considerable time, ranging from several tens of minutes to several hours after the source of the stimulus is cut off. Phosphorescent materials have been reported for use as night-time illumination for vehicle parts, such as wheel valve stem caps, gear shift knobs, or for phosphorescent tabs for illuminating keyholes. They have also been used for night-time illumination of plastic electronic key pads that contain informational indicia attached to the key pads by an adhesive.
Although fluorescent pigments and dyes have been employed in plastic materials suitable for laser marking, it has not previously been considered possible to laser mark indicia on phosphorescent plastic articles. The most common mechanism of laser marking of plastic materials depends on the rapid production of heat in the irradiated portion of the plastic due to the absorption of the laser energy. Many plastics, such as polyethylene, polypropylene and polystyrene, are transparent to laser energy at certain wavelengths, and many other plastics do not absorb laser energy well. In order to laser mark plastics such as these, it is necessary to incorporate laser energy absorbing additives, such as clay, talc, titanium dioxide, carbon black, barium sulfate, and the like, into the polymeric composition. However, it was believed that incorporation of such laser energy absorbing additives into phosphorescent plastic compositions would mask the phosphorescence or, at the least, significantly interfere with the intensity of the phosphorescence and the duration of the afterglow, requiring uneconomically large quantities of these expensive phosphorescent phosphors to overcome the masking effect of the fillers.
Moreover, until recently, the commonly available phosphorescent phosphors, such as sulfide phosphorescent phosphors, were capable of emitting a visible afterglow that lasted only from about 30 minutes to about 2 hours and their use required repeated exposure to a light stimulus in order for the afterglow to be sustained for practical time period. Such sulfide phosphorescent phosphors include CaS:Bi (which emits violet blue light), CaStS:Bi (which emits blue light), ZnS:Cu (which emits green light) and ZnCdS:Cu (which emits yellow or orange light). Because of the short afterglow duration of these phosphors, there were concerns about diminishing the luminescence and afterglow time by adding laser energy absorbing fillers to polymer compositions containing these phosphorescent phosphors.
Recently, however, a new group of phosphorescent phosphors that both absorb and emit light in the visible spectrum has become available (see U.S. Pat. No. 5,424,006). These phosphors comprise a matrix expressed by MAl.sub.2 O.sub.4 (where M is a metal such as calcium, strontium, or barium) doped with at least one other element, such as europium, ytterbium, dysprosium, thulium, erbium, or the like, to form a phosphorescent phosphor (e.g., SrAl.sub.2 O.sub.4 :Eu). These phosphors have a crystalline structure and they are highly chemically stable metal oxides. The new phosphors have an afterglow that is three to more than twelve times brighter than that of the comparable sulfide based phosphorescent phosphors at 10 minutes after stimulation, and 17 to more than 37 times brighter at 100 minutes after stimulation. Moreover, the afterglow of these phosphors is still visibly apparent 15 to 24 hours after stimulation. The suggested uses for these phosphorescent phosphors include phosphorescent articles such as signs, luminous inks, toys, directional markers, and backlighting for liquid-crystal items such as clocks, electrical appliance switches, key hole indicators, and the like, but there is no suggestion that plastic articles containing these or similar phosphorescent phosphors may be suitable for laser marking.
Another group of phosphorescent phosphors containing neodymium and ytterbium ions has also been recently described. These phosphors absorb light in the visible spectrum and emit light in the infrared spectrum (see U.S. Pat. No. 5,220,166) and reportedly can be incorporated into plastic materials during manufacture, into paints used as coatings, or into ink compositions for printing. The infrared light emitted by the phosphor is detected by a photodetector device, such as a bar code reader, credit card/bank card reader, and the like. However, there is no suggestion that indicia can be laser marked on such phosphorescent plastics.